shader nodes to make a realistic looking Amethyst
After a lot of tweaking, I came up with a result that looks decent:


Or how it is more likely to be:


I used a different reference so it's more purple than yours, but that's easily fixed since you seem to have no problem doing the actual crystal material.

Here is the node setup:


I'm sure you know what the top box does since you have already done that. It simply transitions from the transparent crystal material to the opaque white material.

The white material's surface is just a simple glossy add to a transparent shader. I have added it so as to not block any of the light going into the crystal.

I have separated out the purple crystal material into Refraction and glossy so as to have more control. I needed to blur the reflections slightly without adding roughness to the refraction.

The volume scatter is set to white with a really high density (around 50).

The real trick though, was to go to the Light paths and increase the volume bounces from 0 to 5. This causes the light to bounce around and some of it to come back out, causing the volumes to be a lot brighter.



This however increases the render times exponentially. But if you're going for realism, then it's a trade you have to make.

Note: I haven't added a scratch texture to this so the reflections are perfect at the moment. But I have included a procedural scratch texture in the attached blend file, but it isn't connected to the material. It isn't very good but works if it isn't going to be viewed close up.



Hope this helps you!